A sniper is a Sentinel that stands on a high, hard to reach place (by the enemy)and provides accurate, high damaging fire that kills incoming enemies or enemy flag cappers. They can also protect their flag capper by shooting hostile chasers. They can also annoy, and if good can kill, juggernauts shelling their base, causing them to move, thus decreasing their mortar accuracy. Snipers also spot (default left alt key) incoming enemies so their allies can be more prepared to face them or go and attack them. It is possible to be an offensive sniper, but you will probably help your team more by being defensive.
The primary weapon can typically be left up to personal preference, but quick charge and high damage rifles, such as the SAP20, are often more desirable, as the sniper will have little chance to shoot a fleeing capper.
The secondary weapon is almost entirely up to the player, and should be whatever weapon they are most proficient in defending themselves with.
Motion Mine: This mine can be useful for providing that extra defense on the flag. However, its unique trigger mechanism makes it far less useful for self-defense.
Claymore Mine: The claymore can be invaluable in protecting the sniper, but care should be taken, as it only triggers in one direction, and the sniper is not perfectly defended.
T5 Grenade: Useful if the enemy team is harassing the sniper, useless otherwise.
Drop Jammer: The drop jammer is great at hiding the sniper's position from the enemy, and decloaking Infiltrators hoping to catch the sniper off-guard.
Energy Recharge: This pack should only be used in conjunction with either the Phase Rifle or the SAP20, as it allows the sniper to fire off more shots in a shorter amount of time.